{
  "id": "docker-container-networking",
  "title": "It Works Inside the Container, But Not From the Host — Why?",
  "category": "Containers",
  "isolation_type": "shared_namespace",
  "difficulty": "Medium",
  "type": "task",
  "weight": 6,
  "description": "## It Works Inside the Container, But Not From the Host — Why?\n\nA container named `net-lab-web` is already running `nginx:alpine`. It works perfectly INSIDE itself, but `curl http://localhost:8099/` from this Lab's own shell fails.\n\n**Your task:**\n\n1. Confirm the app really does work internally: `docker exec net-lab-web wget -qO- http://localhost:80/`\n2. Confirm `curl http://localhost:8099/` from the host really does fail\n3. Diagnose why (hint: `docker port net-lab-web` shows what ports are actually published — or aren't)\n4. Fix it: remove the container and recreate it WITH the correct port publish flag, so `curl http://localhost:8099/` succeeds",
  "hints": [
    {
      "title": "Checking what's actually published",
      "body": "`docker port <container>` lists every host-port-to-container-port mapping currently configured — an empty result means NOTHING is published, no matter how well the app works internally.",
      "command": "docker port net-lab-web"
    },
    {
      "title": "The fix — port publishing can't be added to a running container",
      "body": "Port publishing is set at container CREATION time, not changeable afterward — you must remove and recreate it with `-p hostPort:containerPort`.",
      "command": "docker rm -f net-lab-web\ndocker run -d --name net-lab-web -p 8099:80 nginx:alpine"
    }
  ],
  "setup_commands": [
    { "command": "docker rm -f net-lab-web 2>/dev/null; true" },
    { "command": "docker pull nginx:alpine" },
    { "command": "docker run -d --name net-lab-web nginx:alpine" }
  ],
  "validation": {
    "commands": [
      { "description": "net-lab-web is running from the correct image", "command": "docker inspect net-lab-web --format '{{.Config.Image}} {{.State.Running}}' 2>/dev/null", "expected_output": "nginx:alpine true", "match": "exact" },
      { "description": "port 8099 is now genuinely published to the container's port 80", "command": "docker port net-lab-web 2>/dev/null | grep 80/tcp", "expected_output": "8099", "match": "contains" },
      { "description": "the published port is actually reachable from the host", "command": "curl -sf http://localhost:8099/ -o /dev/null -w '%{http_code}'", "expected_output": "200", "match": "exact" }
    ]
  },
  "default_namespace": "default",
  "teardown_commands": [
    { "command": "docker rm -f net-lab-web 2>/dev/null" }
  ]
}